Why Evolution Is True (17 page)

No creationist, whether of the Noah’s Ark variety or otherwise, has offered a credible explanation for why different types of animals have similar forms in different places. All they can do is invoke the inscrutable whims of the creator. But evolution does explain the pattern by invoking a well-known process called
convergent evolution.
It’s really quite simple. Species that live in similar habitats will experience similar selection pressures from their environment, so they may evolve similar adaptations, or converge, coming to look and behave very much alike even though they are unrelated. But these species still retain key differences that give clues to their distant ancestry. (A famous example of convergence is the camouflaging white coloration shared by diverse Arctic animals such as the polar bear and the snowy owl.) The ancestor of marsupials colonized Australia, while placentals dominated the rest of the world. Both placentals and marsupials split into a variety of species, and those species adapted to diverse habitats. If you survive and reproduce better because you burrow underground, natural selection will shrink your eyes and give you large digging claws, be you placental or marsupial. But you’ll still retain some characteristic traits of your ancestors.

FIGURE 20
. Convergent evolution of mammals. Marsupial anteaters, small gliders, and moles evolved in Australia, independent of their placental-mammal equivalents in the Americas, yet their forms are remarkably similar.

 

Cacti and euphorbs also show convergent traits. The ancestor of euphorbs colonized the Old World, and that of cacti the Americas. Those species that happened to wind up in the desert evolved similar adaptations: if you’re a plant in a dry climate, you’re better off being tough and leafless, with a fat stem to store water. So natural selection molded euphorbs and cacti into similar forms.

Convergent evolution demonstrates three parts of evolutionary theory working together: common ancestry, speciation, and natural selection. Common ancestry accounts for why Australian marsupials share some features (females have two vaginas and a double uterus, for example), while placental mammals share different features (e.g., a long-lasting placenta). Speciation is the process by which each common ancestor gives rise to many different descendants. And natural selection makes each species well adapted to its environment. Put these together, add in the fact that distant areas of the world can have similar habitats, and you get convergent evolution—and a simple explanation of a major biogeographic pattern.

As for how the marsupials got to Australia, that’s part of another evolutionary tale, and one that leads to a testable prediction. The earliest marsupial fossils, around 80 million years old, are found not in Australia but in North America. As marsupials evolved, they spread southward, reaching what is now the tip of South America about 40 million years ago. Marsupials made it to Australia roughly 10 million years later, where they began diversifying into the two-hundred-odd species that live there today.

But how could they cross the South Atlantic? The answer is that it didn’t yet exist. At the time of the marsupial invasion, South America and Australia were joined as part of the southern supercontinent of Gondwana. This landmass had already begun to break apart, unzipping to form the Atlantic Ocean, but the tip of South America was still connected to what is now Antarctica, which in turn was connected to what is now Australia (see figure 21). Since marsupials had to go overland from South America to Australia, they must have passed through Antarctica. So we can predict this: there should be fossil marsupials on Antarctica dating somewhere between 30 and 40 million years ago.

This hypothesis was strong enough to drive scientists to Antarctica, looking for marsupial fossils. And, sure enough, they found them: more than a dozen species of marsupials (recognized by their distinctive teeth and jaws) unearthed on Seymour Island, off the Antarctic Peninsula. This area is right on the ancient ice-free pathway between South America and Antarctica. And the fossils are just the right age: 35 to 40 million years old. After a find in 1982, the polar paleontologist William Zinsmeister was exultant: “For years and years people thought marsupials had to be there. This ties together all the suppositions made about Antarctica. The things we found are what you’d expect we would have.”

What about the many cases of similar (but not identical) species that live in similar habitats but on different continents? The red deer lives in northern Europe, but the elk, which resembles it closely, lives in North America. Tongueless aquatic frogs of the family Pipidae occur in two widely separated places: eastern South America and subtropical Africa. And we already learned about the similar flora of eastern Asia and eastern North America. These observations
would
be puzzling to evolutionists if the continents were always in their present locations. There would have been no way for an ancestral magnolia to disperse from China to Alabama, for freshwater frogs to cross the ocean between Africa and South America, or for an ancestral deer to get from Europe to North America. But we now know precisely how this dispersal did happen: by the existence of ancient land connections between the continents. (These are different from the huge land bridges imagined by early biogeographers.) Asia and North America were once well connected via the Bering land bridge, over which plants and mammals (including humans) colonized North America. And South America and Africa were once part of Gondwana.

As organisms disperse and successfully colonize a new area, they often evolve. And this leads to another prediction that we made in chapter 1. If evolution happened, species living in one area should be the descendants of earlier species that lived in the same place. So if we dig into shallow layers of rocks in a given area, we should find fossils that resemble the organisms treading that ground today.

And this is also the case. Where can we dig up fossil kangaroos that most closely resemble living kangaroos? In Australia. Then there are the armadillos of the New World. Armadillos are unique among mammals in having a carapace of bony armor—
armadillo
in Spanish means “little armored one.” They live only in North, Central, and South America. Where do we find fossils resembling them? In the Americas, the home of the
glyptodonts,
armored plant-eating mammals that look just like overgrown armadillos. Some of these ancient armadillos were the size of Volkswagen Beetles, weighed a ton, were covered with two-inch-thick armor, and sported spiky balls on tails wielded like a mace. Creationism is hard-pressed to explain these patterns: to do so, it would have to propose that there were an endless number of successive extinctions and creations all over the world, and that each set of newly created species were made to resemble older ones that lived in the same place. We’ve come a long way from Noah’s Ark.

The co-occurrence of fossil ancestors and descendants leads to one of the most famous predictions in the history of evolutionary biology—Darwin’s hypothesis, in
The Descent of Man
(1871), that humans evolved in Africa:

At the time Darwin made this prediction, no one had seen any fossils of early humans. As we’ll see in chapter 8, they were first found in 1924 in—you guessed it—Africa. The profusion of ape-human transitional fossils unearthed since then, with the earliest ones always African, leaves no doubt that Darwin’s prediction was right.

Biogeography not only makes predictions, but solves puzzles. Here’s one involving glaciers and fossil trees. Geologists have known for a long time that all the southern continents and subcontinents experienced a massive period of glaciation during the Permian period, about 290 million years ago. We know this because as glaciers move, the rocks and pebbles they carry with them make scratches in the underlying rock. The direction of these scratches tells us which way the glaciers were moving.

Looking at the scratches in Permian rocks of southern lands, you see strange patterns. The glaciers seem to have arisen in areas like Central Africa that are now very warm and, even more confusingly, appear to have moved from the seas onto the continents. (See the direction of the arrows in figure 21.) Now, this is quite impossible: glaciers can form only in persistently cold climates on dry land, when repeated snows become compacted into ice that begins to move under its own weight. So how do we explain these seemingly willy-nilly patterns of glacial striation, and the apparent origin of glaciers in the sea?

And there is one more part of this puzzle, involving the distribution not of scratches but of fossil trees—species in the genus
Glossopteris.
These were conifers that had tongue-shaped leaves instead of needles
(glossa
is Greek for “tongue”).
Glossopteris
was one of the dominant plants of the Permian flora. For several reasons botanists believe that they were deciduous (shedding their leaves each fall and regrowing them in spring): they show growth rings, indicating seasonal cycles, and specialized features indicating that leaves were programmed to separate from the tree. These, and other traits, suggest that
Glossopteris
lived in temperate areas with cold winters.

FIGURE 21
. Continental drift explains the evolutionary biogeography of the ancient tree
Glossopteris.
Top: the present-day distribution of
Glossopteris
fossils (shaded) is broken up into pieces distributed among the continents, making it hard to understand. The patterns of glacial scratches in the rocks are likewise mysterious (arrows). Bottom: the distribution of
Glossopteris
during the Permian period, when the continents were joined in a supercontinent. This pattern makes sense because the trees surrounded the Permian south pole in an area of temperate climate. And the glacial scratches we see today also make sense, as they all pointed away from the Permian south pole.

 

When you plot the distribution of
Glossopteris
fossils in the Southern Hemisphere—the only region in which they are found (figure 21)—they form a strange pattern, scattered in swatches across the southern continents. The pattern can’t be explained by overseas dispersal, because
Glossopteris
had large, heavy seeds that almost certainly couldn’t float. Could this be evidence for creation of the plant on different continents? Not so fast.

Both of these puzzles are solved when we realize where the present-day southern continents really were during the late Permian (figure 21): joined like a jigsaw puzzle into Gondwana. And when you put together the pieces, the position of glacial scratches and the distribution of trees suddenly make sense. The scratches now all point away from the center of Antarctica, which happens to be the part of Gondwana that passed over the South Pole during the Permian. The snows would have produced extensive glaciers spreading away from this location, making scratches in exactly the observed directions. And when the distribution of
Glossopteris
trees is superimposed on a map of Gondwana, the pattern is no longer chaotic: the patches connect up, running like a ring around the edges of the glaciers. These are precisely the cool locations where temperate deciduous trees would be found.

It isn’t the trees that migrated from continent to distant continent, then: it is the continents themselves that moved, carrying the trees with them. These conundrums make sense in light of evolution, while creationism is at a loss to explain either the pattern of glacial scratches or the peculiarly disjunct distribution of
Glossopteris.